1. Field of the Invention
The present invention relates to a sequential burst mode activation circuit. More particularly, the present invention provides a circuit topology for improving the consistency of performance in the activation and intensity variation of multiple loads. The present invention has general utility wherever multiple loads are employed for intensity variation. Further, the present invention has specific utility where multiple fluorescent lamps, especially multiple cold cathode fluorescent lamps (CCFLs), are employed, for example, in television and computer screens, and in backlights for LCDs (Liquid Crystal Displays).
2. Description of Related Art
Various lighting and dimming circuits and techniques for lighting or dimming lamps or varying intensities of loads are known. One method of dimming a fluorescent lamp, especially as used in a backlight of a liquid crystal display (LCD), is known as a voltage controlled dimming system. The voltage controlled dimming system includes current control and current feedback control. According to the voltage control dimming system, dimming is performed by varying an input voltage to an inverter so as to adjust an output voltage from the inverter (i.e., an application voltage to the fluorescent tube). As the fluorescent tube emits light using discharging energy, when the application voltage to the fluorescent tube is too low, the discharging becomes unstable. For this reason, a large dimming range cannot be achieved by the voltage control dimming system, and the possible dimming ratio is only around 2:1, the dimming ratio being indicative of the dimming range of the lamp system.
Another technique for dimming a fluorescent lamp is the “burst mode” dimming system in which an alternating signal that is supplying power to the lamp is cut with a notch of variable width so as to reduce the power applied to the lamp and thereby provide the desired dimming. The smaller the widths of AC power provided to the lamp, the lower the luminance at which the lamp operates. A common device for providing the ability to vary the width of the pulses are commercially-available pulse-width modulators (“PWM”).
In burst mode dimming, dimming is performed by periodically flashing the light source with a varying time ratio between the light-on duration and light-out duration. Therefore, this system, as opposed to the aforementioned voltage controlled dimming method, offers a large dimming ratio, potentially greater than 100:1, thereby allowing for large variations in luminosity.
U.S. Pat. No. 5,844,540 provides lighting/dimming circuitry for the back light control function in an LCD (Liquid Crystal Display). A “PWM dimmer driving circuit” modulates the magnitude of current to be supplied through an inverter to a fluorescent tube on the back surface of a liquid crystal panel. One goal of this circuitry is to prevent inconsistency of lighting, or occurrence of flicker, between the back light, or fluorescent tube, and the LCD; the other goal is to reduce sound noise. The PWM and inverter circuitry modulates the light source driving means so as to have the ability to periodically flash the light source with varying time ratios between the light on and light off durations, thereby creating different average intensities of light. The light-on duration is determined by a ‘pulse count circuit’ which provides an input for the PWM circuitry; this pulse count circuit counts the number of pulses of the LCD panel horizontal synchronizing signal, and provides for an on-duration that allows for the back light to synchronize it's lighting signal with that of the LCD. Further, the lighting/on-off frequency of the light source is a division of the horizontal driving frequency of the LCD panel's horizontal synchronizing signal, thereby allowing both LCD panel's display and the back light to be in phase with each other. This topology provides a “burst-mode” dimming system but only for a single fluorescent lamp. It further advocates synchronization of backlight lighting with that of the LCD in order to prevent inconsistency of lighting between the LCD and backlight. Note that fluorescent lamps, especially cold cathode fluorescent lamps, are high in impedance when initially powered up. If multiple CCFLs (cold cathode fluorescent lamps) were utilized, synchronization of all lamps with one light source would result in current ripples; these current ripples retard inverter performance and cause flicker. This is because, where multiple CCFLs are synchronized, a power supply needs to provide enough power to turn on all CCFLs concurrently. The instant power delivered from the power supply causes the supply voltage to drop due to its limited dynamic response. Therefore, the use of PWM signals, i.e. “burst-mode” dimming, is not, by itself, effective in providing a solution to flicker/noise in multiple lamp configurations.
One technique used to compensate for flicker or noise in the burst-mode dimming of multiple CCFLs is to place a capacitor in series with the power supply to absorb power surges that cause the current ripples. A drawback of this technique is that, when the lamps turn off in each burst mode cycle, the power supply line, which has an intrinsic inductance, continues to carry current which charges the capacitor, yielding an increase in output voltage.
Prior art teachings with the activation with multiple loads, where the loads are not fluorescent lamps, do not address the flicker or noise problem presented by the activation of multiple lamps.